The present invention relates to an illuminating method and an illuminating apparatus employing a highly directional light source, such as a laser light source, and more specifically, an illuminating method and an illuminating apparatus employing a highly directional light source incapable of emitting a light beam of an uniform cross section intensity distribution due to its excessively high directivity, and capable of achieving uniform illumination or illumination with small unevenness in directional intensity distribution, and to a projection exposure method and a projection exposure apparatus employing such an illuminating method and such an illuminating apparatus.
The significance of improvement of resolution in forming a circuit pattern through the use of a light source capable of emitting light of short wavelength for the exposure of circuit patterns of semiconductor circuits has progressively increased in the degree of integration of semiconductor circuits. The ultraviolet laser, typically, an excimer laser, is a prospective exposure light source for its high energy and for its comparatively high capability of narrowing the bandwidth of spectrum. The cross section intensity distribution of a laser beam emitted by the excimer laser assumes a Gaussian distribution with respect to one direction and a trapezoidal distribution in the other direction owing to its structure. In most cases, the trapezoidal distribution curve is distorted or has a peak in the middle portion, and a laser beam having a uniform trapezoidal cross section intensity distribution is rarely obtained. With lasers other than the excimer laser, such as ultraviolet lasers using second and third higher harmonics, uniformity in the outgoing distribution can scarcely be expected.
Variation in the illuminance distribution over the surface of a reticle illuminated on a semiconductor exposure apparatus must be on the order of .+-.2%. When a lot lens for an illuminating system employing a mercury-vapor lamp as a light source is used in simple combination with a laser light source of a narrow bandwidth for uniform illumination, the coherent laser light forms fine interference fringes on the reticle, and hence uniform illumination cannot be achieved.
Methods of uniform illumination by a laser beam having a Gaussian intensity distribution are disclosed in Japanese Patent Provisional Publication (Kokai) Nos. 56-85724 (first reference) and 59-19332 (second reference). The method of the first reference splits the laser beam into four component beams in a cross section perpendicular to the direction of travel of the laser beam, and superposes the four component beams of the laser beam on an object plane for uniform illumination. To obviate irregular illumination attributable to interference fringes resulting from the superposition of the four component beams of the laser beam, the four component beams are divided into two sets each of two component beams, the interference between the two component beams of one of the sets is suppressed by polarizing the two component beams of the set so that the respective directions of polarization are perpendicular to each other, and the phase of one of the two component beams of the other set is varied within a period of illumination so that interference fringes are caused to disappear by averaging the periods of illumination.
Conventional methods of uniform illumination including that of the first reference are unable to reduce the variation of illumination below a level corresponding to a variation on the order of 3% in illuminance even if the unevenness in illumination attributable to the interference between the two component beams, because the Gaussian distributions are superposed without alteration. These conventional methods are scarcely able to obviate interference fringes and to achieve uniform illumination by varying the phase accurately by a phase angle of .pi. in synchronism with pulse emission within a short time between the former and latter halves of the short pulse when applied to illumination using a laser of short pulse duration, such as an excimer laser employed in the present invention, because the pulse duration is a very short time in the range of several nanoseconds to several tens nanoseconds. Accordingly, when such conventional illuminating methods are employed for illumination, a variation in the illuminance distribution attributable to the interference fringes is added to the foregoing variation on the order of 3% in the illuminance distribution to increase the variation in illuminance distribution to a level on the order of 10% in total. Accordingly, such conventional illuminating methods are not applicable to a field to which the present invention pertains.
The illuminating method of the second reference splits likewise a laser beam having a Gaussian intensity distribution into four component beams and superposes these four component beams for uniform illumination. However, since no means for obviating the interference between the four component beams of the laser beam is provided in the optical path between a point of division and a point of superposition, fine interference fringes are formed when the four component beams are superposed. When this illuminating method is applied to illuminating a reticle on an exposure apparatus, the interference fringes formed on the reticle are printed on a wafer, so that it is impossible to print a circuit pattern formed on the reticle accurately on the wafer.
Japanese Patent Provisional Publication (Kokai) No. 54-111832 (third reference) discloses an exposure method intended to obviate interference fringes resulting from the superposition of the component beams of a laser beam on a reticle and to eliminate speckle noise. This exposure method makes the component beams travel respectively along optical paths having different optical path lengths. Japanese Patent Provisional Publication (Kokai) No. 62-25483 (fourth reference) discloses a method of forming optical paths having different optical path lengths by optical path altering means employing reflection mirrors. This method of the fourth reference is effective with a laser beam having a Gaussian intensity distribution in macroscopically uniformizing the illuminance distribution in part or in microscopically uniformizing the illuminance distribution (reduction of speckle noise), but unable to uniformize the illuminance distribution both macroscopically and microscopically.